The invention relates to boats, such as quartz "diffusion boats" for carrying semiconductor wafers during various high temperature semiconductor processing steps, such as diffusion and thermal oxidation.
It is well known that numerous cleaning, coating and diffusion operations are required in the fabrication of semiconductor wafers, and that often fifty-wafer quartz boats are used, wherein there is three/thirty-seconds (3/32) of an inch center-to-center spacing between the wafers for diffusion operations, whereas for most other cleaning operations, plastic twenty-five wafer carriers, such as one manufactured by Fluoroware, Inc. of Chaska, Minn., have three sixteenths (3/16) of an inch center-to-center spacing between wafers. The plastic wafer carriers also have long vertical side grooves or wafers guides, the bottoms of which are approximately one hundred mils wide and the sides of which are tapered outwardly by approximately two degrees. It is also known that all of the prior quartz boats cause generation of a certain amount of "silicon dust" that is highly undesirable in semiconductor manufacturing operations. The most efficient, highest yield semiconductor manufacturing facilities include expensive equipment and require extensive precautions on the part of workers to maintain the wafer fabrication environment at the highest possible level of purity. For example, expensive laminar air flow systems, and expensive dust filters often are used. The most modern facilities completely eliminate use of tweezers and manual handling of wafers in order to reduce the amount of silicon dust that is produced, and all workers wear clean gowns, hair nets, and in some cases, even face masks. However, due to build-up of static electricity on various wafer handling components, even small amounts of silicon dust can become attached to wafer surfaces or surfaces that touch wafers. For example, silicon dust can become attached to the sides of the grooves of the above-mentioned plastic wafer carriers, and when the wafers are transferred into such carrier, the peripheral portion of the active face of the wafer may slide against such silicon dust, causing defects in one or quite a few peripheral circuits on the wafer. This, of course, reduces yield and increases the overall cost per unit of the circuits being manufactured.
The prior quartz diffusion boats cause generation of silicon dust and quartz dust because the wafer supporting grooves thereof generally have rectangular shapes in which the lower edges of the wafers rest. The wafers tend to "rattle" in these grooves as the boat is moved during semiconductor manufacturing operations. This rattling results in frictional sliding of edges of the wafers against the bottoms of the wafer supporting grooves, thereby causing the breaking off of minute particles of silicon and quartz, which then may adhere to surfaces of semiconductor wafers. This is known to cause defects in integrated circuits. Furthermore, the rectangular shaped wafer supporting grooves of prior quartz diffusion boats do not perfectly fit the rectangular grooves and therefore, it is necessary to have at least three grooves in three separate spaced rails to support each wafer so all wafers in the boat are precisely parallel. This requirement increases the cost of manufacture of diffusion boats, and limits the use of diffusion boats to only one diameter of semiconductor wafers.
Accordingly, it is an object of the invention to provide an improved boat for carrying semiconductor wafers during various semiconductor processing operations.
It is another object of the invention to provide an improved "high temperature" wafer boat which holds large wafers more precisely parallel than prior art diffusion boats to thereby increase the uniformity of various semiconductor processing operations.
It is another object of the invention to provide an improved semiconductor wafer supporting boat such as a diffusion boat which substantially eliminates generation of quartz dust, silicon dust and the like that occurs as a result of "rattling" of wafers as they are being transported in the boat during semiconductor manufacturing operations.
It is another object of the invention to provide an improved boat which operates effectively with minimum friction during use of the wafer transfer apparatus described in the above copending parent application.